1. Field of the Invention
The present invention relates generally to fiber optic data transfer devices and, more particularly, to an optical rotary joint that provides a low loss multi-channel one-directional through-put capability with unlimited bi-directional rotation.
2. Description of the Prior Art
Fiber optic techniques make possible many useful devices for transmitting data with modulated light waves. Many types of fiber materials and designs are available to transfer data modulated optical signals over long distances with very low signal attenuation. In addition to signal attenuations over the fiber length, losses occur at terminations and sharp bends. Signal attenuations not only reduce the signal level, they adversely effect the modulation frequency of digitized data carried by the optical waves. Consequently, it is desirable to minimize the system losses.
Often it is necessary to transfer a modulated light beam between two fibers, one rotating with respect to the other. To accomplish such a transfer, a special component referred to as a Fiber Optic Rotary Joint (FORJ) is required. Such devices must be designed for low optical loss operation and minimum loss variation as a function of rotation angle. Further, it is desirable that the FORJ be passive, not having optical components that require external power, and be bidirectional, permitting the transfer of light beams in both directions.
A configuration of a FORJ of the prior art includes single fibers on either side of the rotary joint having a common axis on the axis of rotation. Such devices may exhibit losses as low as 3 dB. Since the fibers are generally single mode, these devices can provide multi-channel capability only with the utilization of appended complex time multiplexing components. Consequently, the maximum data rate that on-axis FORJs can handle is limited by the data handling capability of the time multiplexers. These systems cease to be viable options when the data rate requirements exceed the capability of the multiplexers. Additionally, the requirements for dual and triple redundancy render the single on-axis fiber rotary joint unacceptable.
To increase the number of fibers that may be included in a FORJ, side-of-the shaft devices have been proposed. In these systems the fibers are positioned off the rotation axis about the rotating shaft, i.e. to the side-of-the shaft. Optical losses associated with such devices increase as the size of the devices increases. To date, the best loss performance achieved with such systems is between 15 and 30 dB. The transfer of data modulated light through rotary joints at bit rates exceeding 2 Giga bit/sec., present day requirements, necessitates a loss through the joint of less than 10 dB. It is therefore apparent that side-of-the shaft rotary joints do not meet the challenge.
Active off-axis rotary joints exist in the prior art which employ photon to electric converters between one set of fibers and an electrical rotary joint and electric-to-photon converters between the electrical rotary joint and the other set of fibers. Though high signal losses are experienced in the conversion processes these are easily off-set with the use of high gain electrical amplifiers. Such systems are extremely complex, require external electrical power for the converters and amplifiers, and are limited to data transfer rates of 5.times.10.sup.7 bits/sec.